Method of drying hardwood



Patented June 12, 1951.

NozDrawing; ApplicationDctolier 16, 1946,. Serial No. 703,460

2 Claims. 1;

Thin 1 wood. boards or sheets, properly dried from a. green state" whileconstrained" against shrinkage llll.Wldth' during*drying; possess certain" desirable" properties: Although there r is a slightcontracti'orrinwidth-uponrelease of the wood at the "endof" the-drying1operation-, the Wood will" not expand nearlyso much as-does like wood, that wasnot constrained during drying, when both afterwards absorb moisture. Consequently, in situations; where. theaswelli'ng ofcwood: uponttaking; up moistn-rea. may, cause trouble, it is of advantage-rte:userwoodthat has. beenidf-ied-iundenconstraintrc Thus, for example, h-ardwood:fioor 'coveringsimadebfi ordinary; normally dried wood from .10 to .20 inch thick, bonded to an underlying sub-floor, frequently buckle or bulge upon taking up moisture; whereas such floors, if made of wood successfully dried under constraint in manufacture, will stay down and remain fiat under the same conditions.

Manufacturing experience has shown that when hardwoods, such as oak and walnut, are dried from a green state under heat and pressure while constrained against shrinkage across the width of the board or sheet (in a ventilated hot press as, for example, shown in my Patent 2,268,477) they are frequently so badly damaged by the drying as to be unfit for use in the manufacture of floor coverings. The damage caused in drying is often not evident in the surface and is revealed only when the wood is sanded and considerable expense has been incurred in fabrication.

The defects in wood caused by drying it under constraint, while being subjected to heat, are generally visible on the surface as dark areas where the wood has been reduced. in thickness due to partial hydrolysis of the cellulosic components of the wood. In such areas some of the cellulose has been converted into soluble sugars, so that the pressence of the highly objectionable hydrolysis is also revealed by an odor as of overheated caramel. The density of the dark hydrolyzed areas exceeds that of the normal wood and, instead of being harder, the wood is easily gouged. As partial hydrolysis over some areas is very frequent, the resultant loss due to damage in drying is very great.

The presence of hydrolysis is frequently not visible from the surface as it may take place only in the interior of the board. In that case the wood is excessively honeycombed and sanding the surface preparatory to finishing may reveal the damage. When this happens, not only is the wood lost because it cannot be used 221- for: flooring, but' considerabler labor-f inr= manuifacture-has alsoibeenslosts I have discovered that thei-damage herein-ide scribed is always associated with thezexpulsion of; free imoisture' in. therwoodiand thati it. can be controlled, Due-1: to; differences structure. of

theewoodwitselfi) in:v different. areas of the same.

board,-,it-;.appears.that in some. areasthe moisture becomes. trappedwsoon-z aftertherxconversion into steam.-begins;,'and=it is the-tpressure of pockets of; high: pressure steam that causes. the hydrole ysise. In .ordento'dry-the .wood successfullywhile: held under constraint. it ..-..is-=.-.ther eforenecessary that-the -.drying..be1.done in :such..a manner that. the. steam pocketseare nottformed... V

Many-trials :weres-made:ineaecommercial operation in endeavors to find those conditions, if there were any, which would prevent the formation of steam pockets in the wood. Attempts were made partially to dry thin oak boards freely, as in air drying or kiln drying, down to the fiber saturation point as revealed by incipient shrinkage, to remove free moisture, and then completing the drying under constraint; but, while hydrolysis under those conditions was not so prevalent, it was still suificiently frequent to cause considerable loss of boards. Additional trials, drying freely still further, proved that the wood' must be dried without constraint until appreciable shrinkage has taken place before hydrolysis is reduced to a satisfactory minimum. It was finally established that the critical point is reachedv when free drying is continued until a shrinkage in width from 1% to 3% has taken place, further drying then being under constraint so that no additional reduction in width occurs, with the exception of that due to elastic contraction upon removal of the constraint.

It appears that when a board is dried without constraint to the state at which shrinkage in width to the extent of from 1% to 3% occurs, the moisture content of the center of the same is at or near the fiber saturation point while the moisture of the surface fibers is below the fiber saturation point; which condition may be used as a basis to determine when drying under constraint should begin, although not as easy to measure as the amount of shrinkage in width.

Drying under constraint may continue until the moisture in the wood is reduced to from 1% to 5%. It is preferable that the boards be dried under constraint to practically bone-dry conditions and then have their moisture raised to the desired 6% to 8%; the addition of moisture being effected. through exposure to high humidities in a humidified enclosure, or through momentary submersion in water, followed by cooling and evaporation of the excess moisture.

The fact that some shrinkage has already taken place at the end of the first drying stage makes it possible quickly and accurately to determine when that stage should end. In other words, it is a simple matter to set a guage for any given lot -of raw. material, which is then dried in' a conventional mechanical drier, or air dried, until it has shrunk to fit the guage and so is ready to be dried under constraint.

The resultant board, dried in two stages as described, possesses maximum width consistent with complete freedom from hydrolysis, it is fiat,

2. The method of drying thin boards of hardwood in which objectionable hydrolysis frequently occurs when the same is dried, from a green state to the state of stable moisture content while held under constraint to prevent shrinkage and heated: which consists in drying such boards without constraint until the free moisture has been removed from the interior thereof and they shrink from 1% to 3% inwidth,.'and*then drying the same under constraint to prevent'shrinkage while being subjected to heat and pressure.

and, when slit and backed with a flexible backing, it is flexible and readily adjusts itself to unevenness in the sub-floor upon which it is laid. When, after bonding to the sub-floor, its moisture is increased, it expands much less than do similar boards dried by conventional procedures, and floor failures due to buckling are practically completely eliminated.

I claim:

1. The method of drying thin boards of hardwood in which objectionable hydrolysis frequently takes place when the same is dried from a green state to the state of stable moisture content, while held under constraint to prevent shrinkage and. subjected to heat: which consists in drying such boards, without constraint, until the free moisture is removed from the interior thereof and the outside thereof is somewhat below the fiber saturation point, .and then drying them under constraint to prevent shrinkage while being subjected to heat and pressure.

ARMIN ELMENDORF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,573,297 Briner et a1 Feb. 16, 1926 1,578,020 Elmendorf Mar. 23, 1926 1,677,963 Ford July 24, 1928 1,903,102 Farley Mar. 28, 1933 1,909,213 Munroe May 16, 1933 2,132,095 Broughton Oct. 4, 1938 2,284,838 Oholm June 2, 1942 OTHER REFERENCES The Kiln Drying of Lumber, by Koehler and Thelen, published 1926 by McGraw-Hill Book 00., New York, pages 42 and 46. I

Drying and Processing of Materials by Carrier r Engineering Corp., published 1929, page 181. 

1. THE METHOD OF DRYING THIN BOARDS OF HARDWOOD IN WHICH OBJECTIONABLE HYDROLYSIS FREQUENTLY TAKES PLACE WHEN THE SAME IS DRIED FROM A GREEN STATE TO THE STATE OF STABLE MOISTURE CONTENT, WHILE HELD UNDER CONSTRAINT TO PREVENT SHRINKAGE AND SUBJECTED TO HEAT: WHICH CONSISTS IN DRYING SUCH BOARDS, WITHOUT CONSTRAINT, UNTIL THE FREE MOISTURE IS REMOVED FROM THE INTERIOR THEREOF AND THE OUTSIDE THEREOF IS SOMEWHAT BELOW THE FIBER SATURATION POINT, AND THEN DRYING THEM UNDER CONSTRAINT TO PREVENT SHRINKAGE WHILE BEING SUBJECTED TO HEAT AND PRESSURE. 